High-Speed Downlink Packet Access (HSDPA, also known as High-Speed Data Packet Access) is a third generation (3G) mobile communications system in the High-Speed Packet Access (HSPA) family and is provided to optimize downlink performance. HSDPA allows networks, like 3G networks based on the Universal Mobile Telecommunications System (UMTS), to have higher (downlink) data transfer speeds and capacity.
Wideband Code Division Multiple Access (WCDMA) UMTS communication networks usually comprise a plurality of Radio Network Controllers (RNCs), each of the RNCs being connected to a plurality of base stations (radio transceivers or radio access nodes), also called Node Bs or NodeBs, via a corresponding Transport Network (TN). The RNCs are typically connected with each other via a core network. The Node Bs wirelessly communicate with communication devices, e.g. mobile terminals, which are usually called User Equipments (UEs) in terms of UMTS.
In a HSDPA network, usually there are two main sources of capacity limitation: First, the radio link that transmits information over the air between one of the base stations (Node Bs) and one of the UEs, and (2) the transport link or TN link that connects one of the Node Bs to the associated RNC. Over the Internet, the Transmission Control Protocol (TCP) provides efficient congestion control mechanisms. However, TCP is not suitable to resolve congestion situations between the RNC and the UE, since the Radio Link Control (RLC) protocol in Acknowledged Mode (AM) provides reliable transmission between the two. This means that the RLC AM protocol retransmits the lost packets between the RNC and the UE. Thus, the TCP does not experience packet losses and does not initiate its congestion avoidance mechanism, i.e., it continues sending more packets into the network.
When the TN is the bottleneck in the HSDPA system, solutions have been introduced to handle the TN level congestion situations and these mechanisms are widely deployed in real HSDPA networks today. The Iub (in terms of UMTS, the Iub interface interconnects the RNC to the Node B) Framing Protocol (FP) has been introduced to be responsible for congestion detection. Standardized signaling messages are also available to inform the RNC about the TN congestion. For example, a detailed description of the TN congestion control mechanism, as well as the Iub FP can be found in “HSPA Transport Network Layer Congestion Control” by Szilveszter Nádas, Sándor Racz and Pál L. Pályi, which was published as a book chapter in “Handbook of HSDPA/HSUPA Technology”.
In HSDPA, the RLC AM protocol is responsible for reliable data transmissions in Layer 2 between the RNC and the UE. This is described in 3GPP TS 25.322, Radio Link Control (RLC) protocol specification. Each RLC AM entity consists of a transmitting and a receiving side. An AM RLC entity can act either as a Sender or as a Receiver depending on the elementary procedure, where the Sender is the transmitter of the RLC AM Protocol Data Units (PDUs) and the Receiver receives and processes the transmitted RLC AM PDUs.
Due to the reliable transmission of RLC AM, it provides seamless handover and channel switching operations. Data packets lost between the RNC and UE due to bad radio or TN link conditions are retransmitted, thus non-congestion related packet loss is avoided, which is beneficial for TCP in terms of utilizing network capabilities. On the other hand, RLC retransmissions can increase the round trip time (RTT) significantly. In addition, reliable packet delivery of RLC makes it impossible to drop or re-order packets (e.g., to indicate TN or radio congestion).
A HSDPA congestion control mechanism is conceivable using which TN congestion is detected at Iub FP level. This may be referred to as AQM Based Congestion Control (ABCC). When TN congestion is detected the application level TCP is informed about this congestion event. In order to inform the TCP about the TN congestion an application level Internet Protocol (IP) packet is dropped. The lost (dropped) packet is then retransmitted over the TN by the RLC protocol. Thus, a lost packet over the TN (lost RLC PDU) does not result in RLC Service Data Unit (SDU) loss because of the retransmission of the RLC AM protocol. That is, the transport network congestion is detected using the current HSDPA flow-control as described in “HSPA Transport Network Layer Congestion Control” by Szilveszter Nádas, Sándor Racz and Pál L. Pályi. However, instead of shaping the flow in the RNC, the application level TCP is notified about TN congestion and the TCP handles the TN congestion.
Further, Quality of Service (QoS) differentiation within the same bearer in the base station is desirable in HSDPA networks. QoS differentiation aims to ensure priority of important or delay sensitive data traffic during scheduling. For instance, the delay sensitive and small-sized Voice over IP (VoIP) frames should be prioritized over File Transport Protocol (FTP) packets in case of a full buffer situation of the base station.
In HSDPA networks, QoS differentiation within the same bearer in the Node B is an unsolved problem. Since in the Node B all packets are within the RLC AM loop, the dropping (or prioritization) of the RLC PDUs is not possible. Even if the order of the RLC PDUs in the queue of the Uu scheduler (in terms of UMTS, the Uu interface is the radio interface between the radio access network and the mobile terminal) in the Node B could be changed, the UE-side RLC layer would not deliver data to higher layers, since it provides in-sequence delivery of IP packets to higher layers (see 3GPP TS 25.322, Radio Link Control (RLC) protocol specification).
In HSDPA, currently there is no solution for supporting QoS differentiation within the same bearer in the Node B. Since the RLC AM protocol provides ordered delivery of packets to higher layers, the problem is how to change the order of the RLC PDUs in the Node B such that e.g., RLC SDU-2 (transmitted after RLC SDU-1) is delivered earlier to higher layers than RLC SDU-1.